In many situations, it is desirable to permanently moor vessels in the ocean, such as storage vessels to receive and store crude oil from an offshore oil field. Such storage vessels are usually extensively modified tankers or barges. In mild environments the storage vessel may be moored by bow hawsers to a single anchor leg mooring or other conventional mooring system. However, storage vessels are frequently located far off shore in severe environments, and, because the storage vessel must remain moored even in storms, high mooring forces are imposed on the mooring system. If the storage vessel is to remain permanently moored, the mooring system must be designed to withstand the highest forces imposed by the most severe environment at the site. To lessen corrosion and wear, it is desirable to have mechanical components, such as mooring and cargo swivels, located so that they will not be subjected to continuous salt water immersion or alternate wetting and dry action which may cause failure of seals and bearings. Because the mooring is permanent, it is further desirable in certain instances to locate swivel seals and bearings where they can be conveniently inspected and maintained.
Several suitable permanent moorings for storage vessels have been of the single anchor leg mooring design, for example see U.S. Pat. Nos. 3,641,602, 3,614,869, and 3,708,811. Other permanent moorings for storage vessels have been of the catenary anchor leg design, for example see U.S. Pat. Nos. 3,538,880 and 3,823,432. However, in both types of such moorings the buoy, located at the water surface, is subjected to high wave forces which increase peak mooring forces. In the single anchor leg mooring the mooring swivel and fluid swivels located beneath the water surface must be removed and brought to the surface for maintenance. In the catenary anchor leg mooring, the anchor system is very expensive, especially in deep water, and the underwater cargo hose system requires frequent maintenance. Other types of permanent mooring systems which employ a yoke type connection are disclosed in U.S. Pat. Nos. 2,882,536 and 3,908,212.